Infrared and terahertz near-field microscopy

Schematics of scattering-type near-field microscopy.

Scattering-type near-field infrared microscopy (s-SNIM) allows the investigation of the optical properties of a given structure with resolution beyond the diffraction limit of E. Abbe. The optical properties of a material underneath a sharp tip can be deduced from the scatterd light. Here, the spatial resolution is only limited by the diameter of the tip which is typically in the 10 nm range. 

s-SNIM investigation of a GaAs superlens: an AFM tip is illuminated by p-polarized FEL radiation, the scattered light yielding information about the local near-field of the sample. Gold stripes (width 2 μm, spacing 10 μm) are imaged through the three-layered system. s-SNIM research topics s-SNIM is a powerful method for investigating the electronic properties of nano-objects like quantum dots, nanocrystals, nanowires, ferroelectric domains, local structures arising from phase transitions etc. Here the FEL provides unique research opportunities allowing us, e.g., to access directly the electronic levels in the conduction band of a single quantum dot. Another interesting application is the development of superlenses. We have recently demonstrated a GaAs-based superlens, whichs allow for imaging of small objects located on the opposite side of the superlens at mid-infrared and THz wavelengths far beyond the diffraction limit.  Images of narrow gold stripes have clearly revealed the sub-wavelength imaging capabilities of such superlenses as well as the resonant character of the wavelength dependence. Recently s-SNOM nanospectroscopy based on a commercial instrument from Neaspec/Attocube with a nano-FTIR module and broadband MIR source has been combined with NIR and THz excitation from a fiber laser and FELBE, respectively. THis allows for multicolor pump-probe studies on the nanoscale.

Publications

A. Luferau, A. Pashkin, S. Winnerl, M. Obst, S. C. Kehr, E. Dimakis, T. V. A. G. de Oliveira, L. M. Eng and  M. Helm, Time-resolved nanospectroscopy of III–V semiconductor nanowires, Nanoscale Adv. 7, 3692 (2025).

A. Luferau, M. Obst, S. Winnerl, A. Pashkin, S. C. Kehr, E. Dimakis, F. G. Kaps, O. Hatem, K. Mavridou, L. M. Eng, and M. Helm, Hot-Electron Dynamics in a Semiconductor Nanowire under Intense THz Excitation, ACS Photonics 11, 3123 (2024)

D. Lang, L. Balaghi, S. Winnerl, H. Schneider, R. Hübner, S. C. Kehr, L. M. Eng, M. Helm, E. Dimakis, and A. Pashkin, Nonlinear plasmonic response of doped nanowires observed by infrared nanospectroscopy, Nanotechnology 30, 084003 (2019)

D. Lang, J. Döring, T. Nörenberg, A. Butykai, I. Kezsmarki, H. Schneider, S. Winnerl, M. Helm, S. C. Kehr, and L. M. Eng, Infrared nanoscopy down to liquid helium temperatures, Rev. Sci. Instrum. 89, 033702 (2018)

M. Fehrenbacher, S. Winnerl, H. Schneider, J. Döring, S. C. Kehr, L. M. Eng, Y. Huo, O. G. Schmidt, K. Yao, Y. Liu, and M. Helm, Plasmonic Superlensing in Doped GaAs, Nano Lett. 15, 1057 (2015)

R. Jacob, S. Winnerl, M. Fehrenbacher, J. Bhattacharyya, H. Schneider, M. T. Wenzel, H.-G. von Ribbeck, L. M. Eng, P. Atkinson, A. Rastelli, O. G. Schmidt, and M. Helm, Intersublevel spectroscopy on single InAs-quantum dots by terahertz near-field microscopy, Nano Lett. 12, 4336-4340 (2012).

S. C. Kehr, Y. M. Liu, L. W. Martin, P. Yu, M. Gajak, S.-Y. Yang, C.-H. Yang, M. T. Wenzel, R. Jacob, H.-G. von Ribbeck, M. Helm, X. Zhang, L. M. Eng and R. Ramesh, Near-field examination of perovskite-based superlenses and superlense-enhanced probe-object coupling, Nature Communications 2, 249 (2011).

R. Jacob, S. Winnerl, H. Schneider, M. Helm, M. T. Wenzel, H.-G. v. Ribbeck, L. M. Eng, and S. Kehr, Quantitative determination of the charge carrier concentration of sub-surface implanted silicon by IR-near-field spectroscopy, Opt. Express 18, 26206-26213 (2010).

S. C. Kehr, M. Cebula, O. Mieth, T. Härtling, J. Seidel, S. Grafström, L. M. Eng, S. Winnerl, D. Stehr, and M. Helm, Anisotropy Contrast in Phonon-Enhanced Apertureless Near-Field Microscopy Using a Free-Electron Laser, Phys. Rev. Lett. 100, 256403 (2008).

H.-G. von Ribbeck, M. Brehm, D. W. van der Weide, S. Winnerl, O. Drachenko, M. Helm, and F. Keilmann, Spectroscopic THz near-field microscope, Opt. Express 16, 3430 (2008). 

S. Schneider, J. Seidel, S. Grafström, L. M. Eng, S. Winnerl, D. Stehr, and M. Helm, Impact of optical in-plane anisotropy on near-field phonon polariton spectroscopy, Appl. Phys. Lett. 90, 143101 (2007).